This invention was made or conceived in the performance of contract number DE-AC06-84-RL-10436 with the U.S. Departmentof Energy.
This invention relates to an improved rail assembly for use with a bridge crane in a radioactive environment, such as a "hot cell" from where nuclear fuel assemblies are repaired or reconstituted.
Nuclear waste disposal facilities typically have special rooms, known as "hot cells", where the radioactive fuel in the fuel rods of used nuclear fuel assemblies may be reprocessed. Such hot cells typically include bridge-type cranes for lifting the long, rectangular fuel assemblies out of shipping casks and lowering them onto special work areas, where workers manipulae individual fuel rods with robotic arms visually monitored through lead-shielded glass.
The bridge cranes used in such hot cells are similar in many respects to conventional bridge cranes that are capable of moving a cable and hook along both an X-axis and a Y-axis. Specifically, such cranes include a first electrically operated trolley chassis that carries the hook and cable and whose wheels ride upon a pair of parallel, beam-type rails in an X-axis direction. The beam-type rails are orthogonally disposed over a pair of mutually opposing support walls oriented in a Y-axis direction. The ends of the parallel beam-type rails are inn turn mounted onto second and third trolleys, each of which rides upon a rail assembly mounted along the crown of one of the mutually opposing support walls. The X-axis movement of the cable and hook of the crane afforded by the first trolley over the beam-type rails, and the Y-axis movement of the beam-type rails afforded by the second and third trolleys over the opposing support walls allows the crane hook to be positioned over any desired point on the floor of the cell.
Each wall-type rail assembly is generally formed by a rail having an I-shaped cross-section which is mounted on the floor of an L-shaped recess in the upper portion of its respective wall. A power track is mounted on the side wall of each L-shaped recess parallel to the I-beam-shaped rail. Each trolley chassis includes an electrical connector which slidably fits into a slot in the power track. To prevent the trolleys from falling off their respective rails during a seismic disturbance, each trolley chassis includes angular brackets which capture the underside of the load-bearing top flange of its respective Ishaped rail.
While such prior art bridge cranes satisfactorily perform their intended mechanical function of lifting, moving, and lowering the heavy fuel assemblies to desired positions within within the hot cell, the applicant has observed that the mechanical configuration of the rail assemblies used in such prior art cranes creates significant problems whenever the hot cell is periodically subjected to a decontamination procedure. Such decontamination of the hot cell becomes necessary as a result of the inadvertent release of fine, highly radioactive particles in the air of the cell due to the rupture or breakage of a fuel rod. Such dust-like, airborne particles of radioactive fuel render the radiation level in the hot cell unacceptably high. The decontamination is carried out by spraying all the surfaces within the hot cell with a decontamination liquid which chemically captures and rinses away the radioactive elements present in the dust-like particles of fuel.
However, the many crevice regions inherent in the prior art rail assemblies provide many areas for the dust-like particles of nuclear fuel to collect which are difficult to spray directly with such a decontamination liquid. Some particularly difficult regions to decontaminate include the areas between the rails and the sides of the L-shaped recesses which house them, the regions between the angular, seismicclamp brackets and the upper flange of the rails, and the interior of the power tracks. Even when these areas are sprayed directly with decontamination liquid, they provide numerous "crud traps" where small puddles of the liquid can accumulate and eventually evaporate, thereby leaving deposits of concentrated radioactive material.
Still another deficiency of such prior art assemblies is the fact that they are relatively difficult to construct. In such assemblies, the I-beam shaped rail is mounted over the top surface of a bed plate that is cast in place on the floor of the L-shaped recess when the concrete that forms the upper portion of the wall is poured. Experience has shown that it is very difficult to maintain the top surface of the rail-supporting bed plate both flat and level as the concrete forming upper portion of the wall is poured. Any resulting irregularities interfere with the crane trolley's ability to smoothly travel along the rail during operation. These effects become more serious when very heavy loads are carried and handled by the crane.
Clearly, what is needed is an improved rail assembly for use with the trolley of a bridge crane which provides a minimum number of "crud traps" for airborne or liquidborne radioactive contaminants to collect and settle. Ideally, such a rail assembly should provide a means for positively maintaining the wheels of the trolley over the load-bearing surfaces of the rails even in the event of a seismic disturbance or other natural catastrophe. The load-bearing bed of such a rail assembly should be simple and easy to manufacture flat and level within small tolerances. Finally, the design of the improved rail assembly should be compatible with any commercially available trolley chassis to avoid the necessity of expensive, custom-made components.